1. Field of the Invention
This invention relates to a discharge lamp lighting apparatus, and more particularly, to a discharge lamp lighting apparatus for controlling a voltage of a switching transistor by increasing the output voltage when switching the apparatus on without excessively raising the output voltage during the lighting operation.
2. Description of the Related Art
Conventionally, a voltage resonance type inverter as described below is known as an inverter for converting D.C. power to A.C. power. The inverter of this type includes a parallel voltage resonance circuit and a switching element for turning on and off an input D.C. voltage at high frequencies, for example, 20 to 100 KHz which is higher than the audio frequency and then applying the input D.C. voltage to the voltage resonance circuit. An A.C. voltage induced in the voltage resonance circuit is applied to a load. In the conventional voltage resonance type inverter, the operation frequency (ON/OFF frequency of the switching element) is not controlled. Therefore, the operation frequency in separately excited inverters is generally set constant and the operation frequency of self-excited inverters varies according to the load to be supplied with the A.C. voltage. Such an inverter technology is disclosed in detail in Japanese Patent Publication No. 57-45040, Japanese Patent Disclosure No. 61-2299 and Japanese Utility Model Disclosure No. 62-69396.
In the above inverter, a voltage applied to the switching element varies according to the condition of the load to be supplied with the A.C. voltage. Therefore, the voltage applied to the switching element tends to be excessively high in the inverter in which the load such as a discharge lamp load significantly varies. In order to prevent the switching element from being damaged due to the excessively high voltage, it is necessary to use an expensive switching element having a high withstanding voltage.
A discharge lamp lighting apparatus using an inverter having a switching transistor whose collector-emitter voltage is set to a constant level has been provided (U.S. patent application Ser. No. 284,377). In the discharge lamp lighting apparatus of the above type using a self-excited inverter, a parallel resonance circuit including a primary winding or a secondary winding of the output transformer is coupled at one end to the collector of the switching transistor acting as the switching element and coupled at the other end to the positive terminal of a D.C. power source. Further, a peak voltage detection circuit is coupled between the collector and emitter of the switching transistor and a control circuit having a transistor for an error detector is coupled between the base and emitter thereof. The peak voltage detection circuit is coupled between the negative terminal of the D.C. power source and the control circuit.
In the above discharge lamp lighting apparatus, the collector-emitter voltage of the switching transistor is detected by the peak voltage detection circuit. When the collector-emitter voltage of the switching transistor rises, the potential at a preset detection point of the peak voltage detection circuit rises. Then, time during which the base current of the switching transistor flows via the control circuit becomes short and the ON-time period of the switching transistor becomes short. As a result, energy stored in the primary winding of the output transformer is reduced so that the outputs of the resonance circuit and the output transformer will be lowered.
In contrast, when the collector-emitter voltage of the switching transistor is lowered, the control which is the opposite of that effected in the case where the collector-emitter voltage rises is effected. More specifically, when the collector-emitter voltage of the switching transistor is lowered, the potential at the preset detection point of the peak voltage detection circuit is lowered. Then, the base current of the switching transistor flows via the control circuit for a longer period of time and the ON-time period of the switching transistor becomes longer. As a result, energy stored in the primary winding of the output transformer is increased so that the outputs of the resonance circuit and the output transformer will be increased.
In this way, the collector-emitter voltage of the switching transistor is controlled to a constant level by effecting the negative feedback control in the conventional circuit.
The conventional apparatus in which the collector-emitter voltage of the switching transistor is controlled to a constant level at the starting time and lighting time for the discharge lamp has the following problem. That is, the collector-emitter voltage of the switching transistor and the output voltage of the inverter may be controlled to a constant level at the starting time and lighting time. However, in order to light the discharge lamp, it is necessary to raise the starting voltage or ignition voltage of the discharge lamp, and therefore it becomes difficult to start the discharge lamp when the output voltage thereof is set at a low level. In contrast, if the output voltage is set at a high level, an excessive voltage will be applied in the lighting operation of the discharge lamp. For this reason it becomes necessary to use a large ballast in order to cope with the above problem.